Purification of aromatic carboxylic acids by catalytic hydrogenation generally involves contacting an aqueous solution comprising an impure-acid product containing a desired aromatic carboxylic acid and impurities, such as a crude product made by oxidation of alkyl or other substituted aromatic feed materials, with hydrogen at elevated temperature and pressure in the presence of a catalyst comprising a metal with catalytic activity for hydrogenation disposed substantially on the surface of a solid carrier that is inert to the reactants and substantially insoluble in the liquid reaction mixture under reaction conditions. Hydrogenation of the aqueous solution of impure product permits separation of a purified, solid product from the hydrogenated reaction solution with a greater part, of impurities that affect quality of the desired aromatic carboxylic acid product contained in the remaining mother liquor as a result of hydrogenation either to species with greater aqueous solubility so that they remain dissolved in the mother liquor or to species less detrimental to quality if present in the purified product.
By way of example, terephthalic acid is widely used for the manufacture of polyethylene terephthalate polyesters used to make fibers, films and bottles, among other things, and is commonly made by heavy metal-catalyzed, liquid phase oxidation of para-xylene feed materials. The resulting crude oxidation product typically comprises the desired terephthalic acid and amounts of oxidation intermediates and other by-products, such as 4-carboxybenzaldehyde and p-toluic acid, and colored or color-forming species such as 2,6-dicarboxyfluorenone and 2,6-dicarboxyanthroquinone. Crude product with up to about 5,000 to 10,000 parts per million by weight (“ppmw”) 4-carboxybenzaldehyde is not uncommon, and even amounts as low as 25 ppmw may be or correlate with impurity levels that may be detrimental to color of polyesters. As known from U.S. Pat. No. 3,584,039, purification of such crude and other impure terephthalic acid products by catalytic hydrogenation of an aqueous solution thereof at elevated temperatures and pressures converts 4-carboxylbenzaldehyde to hydroxymethyl benzoic acid, which in turn is converted to p-toluic acid, both of which are more soluble in the aqueous reaction liquid than terephthalic acid. Solid terephthalic acid with reduced levels of 4-carboxybenzaldehyde compared to the starting crude product can be crystallized from the reaction liquid while hydroxymethyl benzoic acid and p-toluic acid resulting from hydrogenation, of 4-carboxybenzaldehyde remain in solution. Hydrogenation of the crude product also converts colored and color-forming benzil, fluorenone and anthraquinone species such as 2,6-dicarboxyfluorenone and 2,6-dicarboxyanthroquinone, to corresponding colorless or less colored hydrogenated compounds. Related purification of impure isophthalic acid products, commonly made by liquid phase oxidation of meta-xylene feed materials, is disclosed in U.S. Pat. No. 4,933,492.
Conventional catalysts for practical commercial applications of such processes commonly comprise palladium carried on an inert, granular carbon support. Carbon supports are readily obtainable and chemically stable in the high temperature, acidic environments of purification reaction processes. However, carbon supports tend to be fragile and carbon-supported catalysts are easily damaged by process flow, pressure, and temperature upsets. Even minor damage can produce fine catalyst particles that can carry over with the product from a purification reactor and contaminate the purified product. In the case of purified terephthalic acid products, this contamination typically is manifested by high particulate contamination levels as indicated by standard measures such as L* values, which indicate grayness on a scale of 100 (corresponding to white or colorless) to 0 (corresponding to black), with values below 98 generally being considered poor for purified terephthalic acid.
More serious damage to carbon-supported catalysts can degrade a catalyst bed so extensively that reactor pressure drop becomes unacceptable. In such cases, the entire catalyst bed must be replaced. Another consequence of fragility of conventional carbon supports is loss of catalyst metals over the lifetime of a catalyst bed due to fines generated during upsets and catalyst loading and maintenance procedures. Spent catalyst beds containing 70% or even less of their initial catalyst metal contents are not uncommon. Loss of catalyst metals not only diminishes catalyst activity and lifetime but also creates a financial penalty from the lost metals themselves, especially in the case of expensive metals such as palladium.
A stronger catalyst support could reduce these difficulties with carbon supports. In that respect, properties such as crush strength and resistance to abrasion, formation of catalyst fines and loss of catalyst metals under conditions of handling, storage and use are important attributes of a support. Improvements in properties, however, are sometimes difficult to achieve without sacrifices in others. Beyond strength and abrasion resistance, utility of a support with particular catalyst metals for particular chemical reactions on a scale and under conditions suited to practical process applications is impacted, often unpredictably, by its activity, or lack of activity, for side reactions and affinity, or lack thereof, for adsorption and other surface phenomena under conditions of use, surface characteristics, such as surface area, pore size and volume, suited to facile and adequate catalyst metal loadings in catalyst preparation and effective reaction rates during catalyst use, and other factors. Titanium dioxide in rutile form, for example, is more strong and abrasion resistant than conventional carbon supports and, despite surface areas of only about 10 to about 40 m2/g as compared to hundreds to a thousand m2/g in the case of carbon supports, aromatic acid purification catalysts with catalyst metals supported on rutile titanium dioxide are known from U.S. Pat. No. 5,362,908. However, U.S. Pat. No. 5,616,792 indicates that color bodies remain after hydrogenation of crude terephthalic acid using the rutile titania-supported catalysts. Thus, despite improved strength and abrasion resistance compared to conventional carbon supports, performance of rutile titania-supported platinum and palladium catalysts in purification of terephthalic acid is inferior to that of catalysts with conventional carbon supports.
U.S. Pat. No. 3,584,039, noted above, describes catalyst metals and supports for purifying impure terephthalic acid by catalytic hydrogenation of impure terephthalic acid in aqueous liquid phase solution at elevated temperature and pressure, with preference given to Group VIII noble metals including ruthenium, rhodium, palladium, osmium, irridium and platinum as catalyst metals and suitable supports described as insoluble in water and unreactive with terephthalic acid at temperatures of at least 200° C., with carbons and charcoals being preferred. The patent reports that silicon carbide is not useable as a support due to high Si content (18,000 ppmw) of a residue remaining after contacting silicon carbide in an aqueous, 10 wt % terephthalic acid solution at 245° C. and elevated pressure for four hours. Commonly assigned U.S. Pat. No. 5,354,898 discloses purification of aromatic carboxylic acids using a carbon-supported hydrogenation catalyst metal such as palladium or rhodium in which purification reaction solution is passed through a bed or layer of non-catalytic particles with high abrasion resistance to reduce carryover of fine catalyst or carbon particles on removal of the solution from a purification reactor. Abrasion-resistant particles described in the patent have attrition loss according to ASTM D 4058-81 of less than 3%; silicon carbide is included in a list of examples.
Silicon carbide, as conventionally used as an abrasive and in refractory materials such as firebrick, rods and tubes, is commonly prepared commercially by fusing sand and coke in an electric furnace at temperatures above 2,200° C. The resulting silicon carbide forms extremely hard, dark, iridescent crystals that are free of porosity, insoluble in water and other common solvents and stable at high temperatures. It is not attacked by acids or alkalis or molten salts up to 800° C. In air, silicon carbide forms a protective silicon oxide coating at about 1200° C. Surface area of conventional silicon carbides typically is about 1 m2/g. Extremely pure forms of silicon carbide are white or colorless and are used in semi-conductors. More recently, U.S. Pat. No. 4,914,070 has reported silicon carbide in the form of porous agglomerates of submicroscopic grains made by heating a mixture of silicon dioxide and silicon at 1100-1400° C. under pressure of 0.1-1.5 hPa in a first reaction zone to generate silicon oxide vapors and contacting those vapors with reactive carbon in a divided state and having a surface area of at least 200 m2/g at 1100-1400° C. in a second reaction zone. The silicon carbides are further described as a carbonaceous substrate covered with silicon carbide crystallized in a face-centered cubic lattice, with specific surface areas of at least about 100 m2/g and color ranging from dark blue to mouse gray or to a dark shade of sea green. The compositions are said to have utility as supports for catalysts for petrochemical and high temperature reactions, such as rhodium or platinum catalysts for conversion of carbon monoxide and unburned hydrocarbons to CO2 and nitrogen oxide to NO2 in catalytic converters for internal combustion-engines, cobalt-molybdenum catalysts for petrochemical hydrotreatments such as hydrodesulphurization and hydrodemetallation, and for controlled oxidations to convert methane and other low molecular weight hydrocarbons to higher hydrocarbons. Related high surface area silicon and other metallic or metalloid refractory carbide compositions, said to be useful as supports for catalysts for chemical, petroleum and exhaust silencer reactions, and their manufacture, are also described in U.S. Pat. No. 5,217,930 and U.S. Pat. No. 5,460,759
U.S. Pat. No. 5,427,761 also describes production of silicon and other metal carbides, generally stated to be useful as catalysts or catalyst supports for chemical and petrochemical industries or for silencers, having BET surface areas of 10-200 m2/g and made by a process in which a reaction mixture of approximately stoichiometric proportions of a degassed carbon with surface area of at least 200 m2/g and a compound of a metal of which the carbide is to be formed and which is volatile under reaction conditions is introduced into a reactor scavenged with a flow of inert gas and heated at 900-1400° C. to volatilize the metal compound, reduce it with carbon and carburize the reduced product, and the result is cooled to a temperature such that the resulting metal carbide does not oxidize on contact with air, with control of inert gas flow to the reactor based on CO content of gas withdrawn therefrom.
Silicon carbide foams with specific surface areas of 10-50 m2/g and made in similar manner from a polyurethane foam as the starting carbon source are described in U.S. Pat. No. 5,429,780 and U.S. Pat. No. 5,449,654, as is impregnation of the silicon carbide with platinum, rhodium or palladium to form a catalyst, and use of the catalyst for oxidation of exhaust gases and in exhaust filters for diesel engines. Silicon carbide foams said to be useful as shaped catalyst supports as for exhaust pipes and such foams with ceria, rhodia and platinum deposited thereon are disclosed in U.S. Pat. No. 5,958,831. U.S. Pat. No. 6,217,841 describes silicon and metal carbides with large specific surface area (20-100 m2/g) and significant open macroporosity made similarly to the process of U.S. Pat. No. 5,427,761 but with a polyurethane or polyacrylonitrile carbon foam as the starting carbon. The metal carbides are said to have utility as catalyst supports for chemical and petrochemical industries although specific reactions and catalysts metals are not disclosed. U.S. Pat. No. 6,251,819 describes silicon carbide foams, preferably made from an organic foam as a starting carbon source, with surface areas of at least 5 m2/g and said to be useful in exhaust silencers. U.S. Pat. No. 6,184,178 reports catalyst supports in granular form essentially made up of silicon carbide beta crystallites having specific surface area of at least 5 m2/g, and usually 10-50 m2/g, and with crush resistance of 1-20 MPa according to ASTM D 4179-88a. The supports are said to be useful for chemical and petrochemical catalytic reactions such as hydrogenation, dehydrogenation, isomerization, decyclization, of hydrocarbides, although specific processes and catalyst metals are not described.
Use of high surface area silicon carbides as supports for catalysts for hydrogenation of impure aromatic carboxylic acids or for similar reactions at the elevated temperatures and pressures and in the extreme acidic environments of such hydrogenation processes is not reported, nor would utility in such processes have been expected from the instability of silicon carbide in terephthalic acid solution as reported in U.S. Pat. No. 3,584,039.